Transistor amplifiers are commonly used building blocks of analog circuits operating at frequencies ranging from audio to radio frequencies (RF). Generally, these circuits are required to produce minimal distortion of the signal they operate on to preserve information carried by the signal. In circuits that implement feedback, a designer has the option to reduce distortion by controlling the loop gain of the circuit at the expense of overall gain. In feed-forward circuits however, other means of improving linearity, i.e., reducing distortion, are required.
As an example, RF circuits are a common application in which one faces these issues. An RF designer is typically compelled to use feed-forward architectures due to the high frequency of operation. The designer must also consider linearity in systems where the modulation scheme is a non-constant envelope, i.e., where there is amplitude variation between the various symbols. Non-constant envelope modulation schemes (e.g., 64-QAM) are becoming more and more prevalent as system designers seek to achieve greater rates of data transmission. Put simply, designing a circuit with sufficient linearity performance is often one of the greatest challenges in RF design.
Compensation for non-linearities and signal distortion in transistor circuits is known. For example, U.S. Pat. No. 6,531,924 discloses biasing circuits that selectively compensate for second or third order distortion. In this reference, mathematical models for the second order transconductance and the third order transconductance of a given transistor are developed for small deviations of voltages and currents. Based on these models, biasing circuits using direct current operation and comprising an operational amplifier, current mirrors and replica transistors of the transistor in which the transconductance is to be cancelled are designed. The large number of components required in making such biasing circuits and the fact that space available on a circuit die is usually quite limited, can lead to circuit layout problems.
Active biasing of power devices for linear operation is also known. For example, U.S. Pat. No. 7,084,705 to Prodanov discloses biasing circuits for Class-AB power amplifiers. These biasing circuits aim at maintaining a fixed ratio of an operating point transconductance to a maximum transconductance over a broad range of temperatures. This is achieved by measuring transconductance at extreme bias points to determine the bias point; however, the Prodanov approach does not lead to canceling specific Nth-order distortion.
It is, therefore, desirable to provide an approach that requires relatively few circuit components and allows cancellation of specific distortion orders.